A typical source of charged particles, such as electrons or positive ions, which is intended for extraction of electrons or ions, includes a plasma chamber made of graphite, stainless steel, aluminum, etc., and an extraction electrode system for extracting charged particles confined in the plasma chamber. The plasma chamber of a typical ion source consists of a top wall, side walls, and a bottom wall. A plurality of permanent magnets forming a multi-cusp magnetic field for confining plasma is provided at the top and side walls. A gas supply opening for the ion source gas and an antenna introduction opening are provided at the top wall, while the ion extraction system having opened outlets for extracting ion beams are provided at the bottom wall. The extraction electrode system works to extract the ion beams through electric fields of plasma, and the system is usually formed of a plurality of electrodes such as a plasma electrode, an extraction electrode, a suppression electrode, and a ground electrode.
It is desirable to provide the aforementioned ion-beam sources that produce high ion current at moderate ion energy, and this requirement leads to the concept of the so-called broad-beam ion source. The ions originate from a low-pressure gas discharge and are extracted by a multi-aperture extraction system. Initially, the development of broad-beam ion sources was associated with ion-beam thrusters for space propulsion. Today, however, the ion-beam sources of this type are also very common in many fields of surface treatment of materials.
The known device nearest to the present invention is an ion beam source based on ICP (inductively coupled plasma) described in http://casetechnology.com/implanter/source.html. This is a source shown in FIG. 1A and FIG. 1B, which illustrate schematic longitudinal and transverse sectional views of the source, respectively. As shown in the drawings, the ion source, which in general is designated by reference numeral 20, has arrays 22a (FIG. 1) and 22b (FIG. 2) of permanent magnets to produce a multi-cusp magnetic field in regions remote from plasma grids 24 and an RF antenna 26 that is formed by wire coil for confining the plasma by lengthening the path of ionizing electrons and reducing their drift to the walls of the working chamber 28 of the ion source 20. The chamber 28 is short in length, relative to its transverse dimensions, and the RF antenna 26 is at an even shorter distance from the extraction electrode 24a of the plasma grids 24, which contains one or more extraction apertures. Reference numerals 24b and 24c designate suppression the electrode and ground electrode, respectively. The RF electric field coupled into the plasma chamber 28 maintains a low pressure (10−5 to 10−2 Torr) discharge. Positive ions are expelled from the discharge by a negatively biased extraction electrode 24a. 
In FIG. 1A, reference numeral 30 designates a magnetic filter which reduces the production of undesired ion species and improves the ion beam quality. High-energy ionizing electrons are confined by the magnetic filter 30 to an ion source region, where the high-energy electrons ionize gas molecules. One embodiment of the magnetic filter uses permanent magnets oriented to establish a magnetic field transverse to the direction of travel of ions from the ion source region to the ion extraction region. In another embodiment, low energy 16 eV electrons are injected into the ion source to dissociate gas molecules and undesired ion species into desired ion species (see U.S. Pat. No. 4,447,732).
The density of a plasma is dictated by the balance between production and loss processes, with the added restriction that to maintain neutrality the ion charge and electron charge densities must be equal. Energetic electrons, which are more useful for ionization, are more easily lost to the chamber walls than the slower ions unless steps are taken to return the fast electrons to the plasma P. It would also be of advantage to allow slow electrons with less than the minimum ionization energy to escape thus reducing the possibility of electron-ion recombination. A strong multi-pole magnetic field surrounding the plasma volume meets these requirements.
Normally, the ion sources of the aforementioned type use Samarium-cobalt permanent magnets of about 1.5 killoGauss.
The antenna 26 is supported by the base has a metallic radio frequency conducting segment mounted directly within the plasma chamber 26 to deliver ionizing energy into the gas ionization zone.
The source gas flow is from 1 to 10 sccm to sustain the plasma between pressure zones within the source 20, which operates between 1×10−5 Torr and 1×10−2 Torr in the chamber 26 depending upon the pumping capacity of the vacuum system for that particular operating gas. Beam current varies according to the pumping speed and the capacitance of the vacuum system. Typical injection beam current is a between a few milliamps and ninety milliamps depending on the gas flow rate and RF power.
The antenna and source are connected via a matching network 32 including variable capacitors and a step down transformer (not shown) to maximize power transferred to the load and tuning a load. The values of the variable capacitors are varied simultaneously until the best impedance match between the impedances seen looking into and out of output terminals of the RF generator 34 is attained.
The electrode assembly, which consists of the aforementioned grids 24a, 24b, and 24c is bounded in close proximity to the ion source 20. The extraction electrode 24a is used for extracting positively charged ions from the source. Ions exiting the source 20 combine downstream to form a broad beam, which is used for ion beam treatment of a silicon wafer (not shown). Individual electrodes 24b and 24c in close proximity to the extraction electrode 24a can be biased to either inhibit or allow backstreaming of neutralizing electrons from beam portions close to the source back to the extraction electrode 24a. Insulators 36 separates beam portions in close proximity to the extraction electrode 24a to inhibit beam crosstalk and an additional suppression electrode common to all beam portions is controllably biased to further enhance control over beam portion intensity. In a typical application, the beam is a circular beam and intensity control is maintained to assure common intensity for a given radii from the beam center.
Specific examples of other known ion sources are also disclosed in a number of patents some of which are mentioned below.
For example, U.S. Pat. No. 4,259,145 issued in 1981 to Harper, et al. discloses reactive ion etching of materials which is carried out using a low energy ion beam of controlled energy and current density. The ion beam is generated with an ion source using a single extraction grid having multiple apertures to obtain high current densities at low ion energies. A reactive gas such as CF4 is introduced into the ion source and ionized to form plasma which acts as the source of ions for the beam. The plasma forms a sheath located adjacent to the single extraction grid such that the ions are extracted from the plasma through the grid apertures and form a low energy ion beam for bombarding the wafers for etching the same. The size of each of the grid apertures is about the same or smaller than the thickness of the plasma sheath adjacent the grid. The ion source is designed to produce an ion current density of about 1 mA/cm2 at a low ion energy of about 10-100 electron volts. This low energy minimizes etching by physical sputtering and allows the chemical component of reactive ion etching to dominate.
U.S. Pat. No. 4,481,062 issued in 1984 to Kaufman discloses an electron-bombardment ion source that includes means defining a chamber for containing an ionized gas together with means for introducing such gas into that chamber. Disposed therein is an anode and an electron-emissive cathode. The potential impressed between the anode and the cathode to effect electron emission at a sufficient velocity to ionize the gas. Also included are means for accelerating ions out of the chamber together with means for establishing a magnetic field within the chamber that increases the efficiency of ionization of the gas by the electrons. Mounted within the chamber is an anode of non-magnetic material that defines an essentially continuous and smooth surface which encloses substantially all of the volume within which the ionization occurs except the exit for the accelerated ions out of the chamber. The entire design is such as to ensure ready removability of the different components for quick and easy cleaning.
However, the method of generation of plasma in the sources described above, including the structures of the above-mentioned patents, in principle, does not allow obtaining plasma of density sufficient for obtaining high ion current densities.
A step forward in the direction of increasing the ion current densities was made with the development of so-called electron-cyclotron resonance sources and electron Hall-drift sources.
For example, U.S. Pat. No. 6,803,585 issued to Glukhoy in 2004 discloses an electron-cyclotron resonance (ECR) type ion beam source for an ion implanter. The apparatus has a sealed plasma chamber in which plasma is excited by microwave radiation of 2.45 GHz in combination with an external magnetic field generated by permanent magnets surrounding the plasma chamber. The magnets cause electron-cyclotron resonance for the electrons of the plasma thus creating conditions for efficient absorption of the microwave energy. The same magnets generate a magnetic field, which compresses the plasma toward the center for confining the plasma within the plasma chamber. The ion source also has a microwave pumping power unit that pumps into the plasma the microwave energy. The RF pumping unit has a unique additional function of RF magnetron sputtering of solid targets converted into a gaseous working medium used for implantation in an ionized form. For obtaining elongated belt-type ion beams (having a width of 1 m or longer), the ion source may contain a microwave pumping system having several output windows arranged in series along the axis of the plasma chamber and on diametrically opposite sides thereof. The windows are continuously cleaned from the contaminants that might precipitate onto their surfaces. A standard-type sand blaster can be used for cleaning the windows.
An example of an end-Hall ion source is a device disclosed in U.S. Pat. No. 4,862,032 issued in 1989 to Kaufman, et al. A plasma-producing gas is introduced into a region defined within an ion source. An anode is disposed near one end of that region, and a cathode is located near the other. A potential is impressed between the anode and the cathode to produce electrons which flow generally in a direction from the cathode toward the anode and bombard the gas to create plasma. A magnetic field is established within the region in a manner such that the field strength decreases in the direction from the anode to the cathode. The direction of the field is generally between the anode and the cathode. The electrons are produced independently of any ion bombardment of the cathode, the magnet is located outside the region on the other side of the anode and the gas is introduced uniformly across the region.
It is long recognized that the inductively coupled plasma (ICP) sources are advantageous for use in the sources of charged particles since these sources are more durable, convenient, and cost-effective devices for plasma generation. Such sources are also used for activating gases needed for cleaning plasma-processing chambers and for incineration (abatement) of harmful gases formed during plasma processing. Application of inductive discharges has an advantage of achieving high density plasma in a wide range of gas pressures with efficient energy transfer to the plasma electrons rather than to the plasma ions as is typical of capacitively coupled RF discharges.
The heart of an industrial ICP source, which is similar to one described above in the first-mentioned reference, is an ICP antenna, which normally comprises a spiral flat coil that is located in the electrically non-conductive upper part of the working chamber and that generates an electromagnetic field that induces plasma in the chamber.
An ICP antenna loaded with plasma has mainly inductive impedance (reactance) that has to be compensated with matching-tuning network (matcher) for maintaining impedance conditions required for efficient transfer of RF power from an RF generator to the plasma-excitation antenna of the plasma source.
A main disadvantage of known industrial ICP in application to ion sources is that they cannot provide high density plasma sufficient for generation of high ion current densities due to relatively weak coupling between the antenna and the plasma.
Another common problem that occurs in ICP used in ion sources, like that shown in FIG. 1, results from a high RF voltage (a few kV) between the terminals of the inductor coil (antenna). High antenna RF voltage requires special means for adequate electrical insulation and leads to considerable capacitive coupling between the coil and plasma. The non-linear electromagnetic interaction between the field of the RF coil and the plasma sheath around the coil immersed into plasma creates a high negative DC voltage in the sheath. The aforementioned negative DC voltage accelerates the plasma ions towards the immersed antenna coil causing its erosion and sputtering that contaminate ion beam and reduce life of the ion source.